1. Field of the Invention
The present invention relates to polyester sheet having excellent heat stability and mechanical strength which are prepared by using aliphatic polyesters with biodegradability and sufficiently high molecular weights and specific melt properties for practical use.
Further, the present invention relates to polyester stretched sheet formed by using the above aliphatic polyesters.
Furthermore, the present invention relates to polyester sheet having low affinity to aromatic substances. More particularly, the present invention relates to a sheet suitable as a material for packaging raw materials, intermediate or finish products, such as food and cosmetics, that contain aroma substances.
2. Discussion of the Background
Recently, with the expansion of the packaging industry, an increasing amount of plastics is being used to form various packaging materials. As a result, people have become very much concerned about the danger that the resulting waste from the increasing amount of plastics may pollute rivers, oceans and soil. To prevent such pollution the development of biodegradable plastics has been desired; for example, poly(3-hydroxybutylate) produced by fermentation methods using microorganisms, blends of general-purpose plastics and starch, a naturally occurring polymer, and the like are already known. The former polymer has a drawback in that it is poor in molding properties because the polymer has a heat decomposition temperature close to its melting point and a raw material efficiency is very bad because it is produced by microorganisms. On the other hand, since the naturally occurring polymer of the latter does not by itself have thermoplasticity, the polymer has defects in molding properties, and is greatly limited in its range of application.
On the other hand, although it is known that aliphatic polyesters are biodegradable, they have hardly been used because polymeric material sufficient enough to obtain practical molded product cannot be obtained. Recently, it has been found that a ring-opening polymerization of .epsilon.-caprolactone produces a higher molecular weight polymer, and proposed to use the polymer as a biodegradable resin. However, the resulting polymer is limited to only special applications because of a low melting point of 62.degree. C. and a high cost thereof. Further, although glycolic acid, lactic acid and the like are polymerized by a ring-opening polymerization of glycolide and lactide thereof to obtain polymers with higher molecular weights so as to be sometimes used as medical fibers and the like, the polymers are not used in great amounts as packaging materials because their decomposition temperatures are close to their melting point and they have defects in their molding properties.
Although most of these are applied to plastic sheet, it is no exaggeration to say that high molecular weight polyesters (referring to polyesters having number-average molecular weights of at least 10,000) generally used for the plastics are limited to polyethylene terephthalete, a condensate of terephthalic acid (including dimethyl terephthalate) and ethylene glycol. Although there are cases of 2,6-naphthalenedicarboxylic acid being used instead of terephthalic acid, there are no reports of trials which obtained polymers with biodegradability.
Therefore, it is safe to say that there has been no concept of trying to make the sheet in practical use by injection molding using biodegradable aliphatic polyesters in which aliphatic dicarboxylic acid was used
One of the reasons why this application concept has not been thought of is felt to be that in spite of the required special molding conditions and physical properties for the above sheet, most of the above-mentioned aliphatic polyesters have melting points of 100.degree. C. or lower even if they are crystalline, and have poor heat stability when melted above that. Of further importance is that the properties, particularly mechanical properties such as tensile strength, of these aliphatic polyesters show markedly poor values even when they are the same level of number-average molecular weight as the above-mentioned polyethylene terephthalete, so just conceiving that the molded articles having required strength and the like would be obtained was difficult.
Another reason seems to be that studies for improving the physical properties of the aliphatic polyesters by increasing their number-average molecular weights have not been sufficiently advanced because of their poor heat stability.
Aroma substances are an important factor for increasing the commercial value of many products, for example, foods, cosmetics, detergents, paints, adhesives, tea, coffee and spices.
Many foods contain very small amounts of various aromatic substances. The proportion of aroma substances contained in a food are specifically fixed so as to provide an aroma characteristic to the food. A variety of aroma substances are added to many commercial products so as to enhance the aromas thereof or add extra aroma thereto, thus increasing the commercial values thereof.
Many organic compounds are known as aroma substances, for example: terpene hydrocarbons such as p-menthane, pinene, d-limonene, myrcene, terpinene, carene, sabinene, and .beta.-caryophyllene; terpene alcohol compounds such as geraniol, nerol, citronellol, terbineol, linalol, menthol, nerolidol and borneol, and esters thereof; terpene aldehyde compounds such as ciral and citronellal; alcohols such as octanol, benzine alcohol and eugenol; esters such as ethyl caproate, amyl benzoate and ethyl cinnamate; and many others.
The products containing such aroma substances are packaged by using packaging materials made of glass, metal or synthetic resins when stored, transported and sold. Particularly, packaging and/or container materials employing synthetic resin sheets and thermally-formed containers are used as simple packaging and/or container materials to pack many commercial products because these packaging materials can be easily produced due to advances in multi-layer and heat-seal techniques and because the packaging and/or container materials are inexpensive, facilitate automatic packaging and decorative printing thereon, and completely barriers oxygen and moisture.
However, many synthetic resin sheets used to form the above-mentioned packaging materials rapidly absorb large amounts of aroma substances added to or originally contained in the products packaged therewith, so the products packaged therewith lose their aroma and commercial values.
Further, because these synthetic resin sheets absorb different aroma substances at different absorption rates, a synthetic resin sheet packaging a product may take up certain aroma components over the other aroma components from the product. If this happens, the aroma of products made up of a certain combination of many aroma substances change, thus significantly reducing the commercial value of the product.
The term "absorption" means a phenomena in which aroma substances move out of a packaged product and dissolve and diffuse into the synthetic resin of the packaging material or a phenomena in which aroma substances in a solution move out therefrom and dissolve and diffuse into the synthetic resin.
The relations between synthetic resins and aroma holding and absorption of aroma substances are described by, for example, Watanabe Wataru et al. in Nihon Shokuhin Kogyo Gakkai-shi, 10, No. 4, p. 118 (1963), a special issue of Shokuhin Kogyo, Shokuhin no Housou to Zairyo, Korin (1980), by Boda Shigeyuki in Japan Food Science, p. 49 (March, 1987), and by Preceedings of Future--Pak '87 (Ryder Association Inc.) Nov. 9-11 (1987).
Various methods related to containment of aroma are known, for example: a method in which a layer to be in contact with a product packaged with the sheet (the innermost layer) is formed of any one or more of polyethylene terephthalate, ethylene-vinyl alcohol copolymer and nylon (Japanese Patent Application Laid-open Nos. 57-163654 and 60-48344); a method in which a mixture of polyester and polyamide is used to form the innermost layer (Japanese Patent Application Laid-open No. 61-64449); a method in which ethylene-vinyl alcohol copolymer is laminated on a corona-treated or flame-treated low-density polyethylene laminated on a paper board (Japanese Patent Application Laid-open No. 63-3950); and a method in which ethylene-vinyl alcohol copolymer is laminated on an adhesive layer formed on polyolefin, the ethylene-vinyl alcohol copolymer being used as a heat-seal layer (Japanese Utility Model Application Laid-open No. 63-21031). However, any of these methods has problems in that the polymer has a higher melting point and poorer heat-seal characteristic and is more brittle than polyolefin.
A different type of method is proposed (for example, in Japanese Patent Application Laid-open Nos. 59-174348 and 59-174470), in which method the resin forming the innermost layer has been mixed with aroma substances that are expected to be absorbed therein. However, when food aroma substances for food are mixed with the innermost layer resin, the aroma substances deteriorate due to heat, or the mixing proportions of the aroma substances change, thus resulting in an aroma different from the aroma of the product to be packaged.
Polyolefin resins, such as polypropylene, intermediate or low-pressure polyethylene, high-pressure polyethylene, or ethylene-vinyl acetate copolymer (referred to as "EVA" hereinafter), which have good heat-seal characteristics and good moisture blocking characteristics, strongly absorb terpene hydrocarbons but do not substantially absorb alcohol or ester aroma substances; in particular, they barely absorb alcohol aroma substances. Therefore, the packaging materials formed of polyolefin resins are liable to change the proportions of the aroma substances contained in the products packaged therewith and thus change the aroma of the products, thereby significantly reducing the commercial value of the products.
The permeation or diffusion of the aroma substances contained in a film to the outside of the film can be prevented by laminating aluminium foil on the inside surface of the film. Recently, instead of aluminium, a plastic film having a good gas-barrier characteristic is laminated on the film.
However, even if permeation and diffusion of aroma substances is prevented, absorption of aroma substances of a packaged product into the resin forming the innermost layer of the packaging material is inevitable as long as a food containing aroma substances is in contact with the surface of the innermost material, which surface easily absorbs the aroma substances.
Another important characteristic required for a packaging material is a characteristic for sealing a product packaged therewith. For a good sealing characteristic, a film resin having good heat-sealing characteristics is used.
Known general-purpose resins, such as polypropylene, intermediate or low-pressure polyethylene, or high-pressure polyethylene, having excellent film-formabilities easily absorb large amounts of terpene hydrocarbon aroma substances. Therefore, these resins can not be suitably used as the above-described resins which do not substantially absorb aroma substances.
An ethylene-vinyl alcohol copolymer having a vinyl alcohol component (referred to as "EVOH" hereinafter) significantly prevents absorption of aroma substances thereinto but has a poor heat-seal characteristic. Thus, EVOH is not very suitable as an inside-laminate material of a film-type packaging material. As an EVOH having a vinyl alcohol content of less than 25 mol % lacks sufficient gas barrier characteristics and because an EVOH having a vinyl alcohol content of more than 75 mol % cannot be extrusion-formed in substantially the same manner as polyolefin, an EVOH having a vinyl alcohol content of 25-75 mol % is normally selected in order to achieve good gas barrier characteristics. Such an EVOH can be extrusion-formed in substantially the same manner as polyolefin resins and can substantially prevent gas permeation. In fact, such an EVOH is often used for those purposes. However, because such an EVOH lacks the good heat-sealing characteristics required for an inside-laminate material of a film-type packaging material, it has hardly ever been used as an inside-laminate material of a heat-seal packaging material.
On the contrary, aromatic polyesters have good characteristics for preventing absorption of aroma substances thereinto but poor heat-sealing characteristics. Therefore, the aromatic polyesters have hardly been used as inside-laminate materials for heat-seal packaging materials.
Thus, it has become apparent that there is a great need for development of an inside-laminate material for a film-type wrapping material which is excellent in these two contradictory characteristics, that is, the characteristic for preventing absorption of flavor substances and the characteristic for heat-sealing.
Due to the broader use of plastic wrapping materials, there is a possibility that the resulting large amount of plastic waste may pollute rivers, oceans and soils. To prevent this possible pollution, there is a great expectation regarding the development of plastics which can be biologically degraded.
An object of the present invention is to provide an aliphatic polyester sheet formed of a material containing as a main component a biodegradable aliphatic polyester, the material having molecular weights high enough for practical use of the sheet, which sheet has substantial biodegradability and excellent mechanical properties, for example, excellent heat stability and tensile strength.
Further, another object of the present invention is to provide a polyester stretched sheet formed by using the above aliphatic polyester.
Furthermore, another object of the present invention is to provide a polyester sheet which is formed by using the above aliphatic polyester and which can be suitably used as an inside-laminate material for a film-type packaging material having both good characteristics for preventing absorption of aroma substances and good characteristics for heat-sealing.